1 . Field of the Invention
The present invention relates to a retro-focus type lens and, in particular, to a retro-focus type lens used in a color liquid crystal projector which combines a plurality of images having different kinds of color information by a compound prism and then projects, under magnification, the resulting composite image onto a screen surface.
2. Description of the Prior Art
There have conventionally been proposed various kinds of color liquid crystal type projectors which projects, in an optically overlaying fashion onto a screen surface through a projection lens, images displayed on a plurality of liquid crystals (liquid crystal light valves).
An example of conventional color liquid crystal type projectors will be explained with reference to FIG. 11.
As shown in FIG. 11, this conventional color liquid crystal type video projector 50 comprises a light source 51; a pair of lens arrays 52a and 52b; first and second dichroic mirrors 53a and 53b for separating three primary color light components; first, second, third, and fourth total reflection mirrors 54a, 54b, 54c, and 54d; three liquid crystal display panels 55a (for red light), 55b (for green light), and 55c (for blue light); a dichroic prism 56 for combining three primary color light components together; and a projection lens 57 disposed on the exit side of the dichroic prism 56.
The light source 51 is a high-luminance white light source such as halogen lamp or metal halide lamp.
Usually, on the exit side of the light source 51, a UV/IR cut filter for cutting ultraviolet and infrared rays is disposed, whereas cooling means for cooling the light source 51 and the vicinity thereof by air is provided.
The first dichroic mirror 53a is a blue/green light reflecting mirror, whereas the second dichroic mirror 53b is a green light reflecting mirror. Each of these mirrors comprises a glass substrate and a dichroic film coated thereon which is made of a dielectric multilayer film having a spectral characteristic for reflecting a predetermined primary color light component as a mirror.
Each of the three liquid crystal display panels 55a, 55b, and 55c is constituted by a liquid crystal display device of a twisted nematic type (TN, STN, or TFT type) or the like, and displays an image in response to its corresponding liquid crystal signal from a non-depicted liquid crystal driver, thereby modulating its incident primary color light component in terms of luminance.
Further, the dichroic prism 56 is constituted by four pieces of rectangular prisms joined together. Its two orthogonal joining surfaces are formed with dichroic films made of dielectric multilayer films respectively having spectral characteristics as red and blue light reflecting mirrors. Accordingly, the dichroic prism can combine three primary color light components of red, green, and blue into a single white luminous flux.
A field lens 58a and a relay lens 58b are disposed in a system for blue light; whereas field lenses 59a, 59b, and 59c are respectively disposed in front of the liquid crystal display panels 55a, 55b, and 55c.
The projection lens 57 used in thus configured liquid crystal video projector 50 projects the combined three primary color light components onto a screen disposed at a predetermined distance, such that a full-color image can be projected onto the screen under magnification.
The projection lens 57 is required to have the following characteristic.
First, since light components are combined by the cross dichroic prism 56, the projection lens 57 is required to minimize chromatic aberration, have long back focus, and be telecentric. Also, in order to obtain a large projection image in a short projection distance, it is necessary for the projection lens 57 to have a wider angle. Further, in order to project the square liquid crystal display panels 55a, 55b, and 55c without distortion, the projection lens 57 should lower its distortion.
Conventionally known as a wide angle lens having a long back focus is a retro-focus type lens used as a wide angle lens of a single lens reflex camera.
The retro-focus type lens used as a wide angle lens of the conventional single lens reflex camera has a hard exit angle, thus failing to be telecentric. Accordingly, in the case where such a retro-focus type is used in a color liquid crystal type projector, when color light components are combined by the cross dichroic prism 56, color shading may occur on the screen. Therefore, the conventional retro-focus type lens cannot be used as the projection lens of the color liquid crystal type video projector as it is.
Known as the method of focusing the conventional projection lens are a method in which the whole lens system is moved together, and a method in which a plurality of lens groups are moved and floated so as to correct the image surface inclination caused by focusing.
In these conventional focusing methods, however, the lens barrel system at the attachment section between the lenses and their mount increases its size, and the configuration of the apparatus is complicated.